Tsetse flies, Glossina spp., are obligate blood feeders and the vectors in Africa of the trypanosomes that cause sleeping sickness in people and 'nagana' in cattle. More than 60 million people are at risk from human sleeping sickness and severe epidemics occur in many parts of Africa. Animal trypanosomiasis prevents the use of domestic animals for agriculture in c.1 1.5 million km2 of sub-Sahara Africa, thus representing an enormous barrier to agricultural and economic development. The distribution of tsetse arises from interactions among historic events, tsetse dispersal capacity, natural barriers to dispersal, host distributions, climate, and adaptation to prevailing conditions. Among these factors, it was shown that tsetse distribution and abundance are most strongly correlated with temperature and saturation deficit. Moreover, most tsetse fly populations were shown to be strongly differentiated genetically. Because of these preliminary data and because tsetse populations are distributed discontinuously, the likelihood exists of genetic adaptation to local conditions. Do tsetse responses to temperature and water stress correspond to environmental variation in temperature and saturation deficit? Does genetic differentiation among populations correspond with differential responses of populations to temperature and water stress? It is important to answer these and related questions to deploy effectively area-wide control measures such as the sterile insect technique. We now propose to combine molecular techniques, physiological measurements, satellite data and ecological theory to address key questions about the genetics, physiology and geographical distribution of G. morsitans centralis, G. m. morsitans, and G. pallidipes in East and southern Africa. By adopting an integrated approach to understanding the distribution of tsetse in these regions, we shall achieve a comprehensive picture of tsetse fly adaptation to its environment, rates of gene flow (hence dispersal), and predictive models of distribution and abundance.